Now as it happened, David Sparvell G4FTC pointed out to me that there are some sort of transmissions audible on 1810 kHz after dark. David and myself did some work on this signal; we think it's a hyperbolic system very similar in characteristics to HiFix but with some subtle differences- for example, the transmissions are simultaneously on 2 frequencies 815 Hz apart- which could be for lane identification. You can hear two sites quite well with differing fading patterns indicating significantly diffrent locations; and a third can just be heard when the signal is on a peak.
We speculated about the source, as Russia seemed a good bet on two counts- it explains the signal coming up after dark, and its possible they still use MF hyperbolic systems.
I did some trawling on the web and came up with these:
"...there are Russian BRAS and RS-10 navigation systems spreading all over the topband. They sound like a series of dashes.
As these signals are used for hydrographic purposes, they are more active during summer time."
- Vaino Lehtoranta, OH2LX writing on October 1998 on the top band contesting message board. And, from the Spooks Newsletter, another quote from Vaino:
>"...frequency is 3756 kHz with much reduced carrier. It is modulated by tone which spreads up and down symmetrically in about 0.82 kHz steps. Because this is exactly what Bras and Rs-10 transmit, these must have something to do with those systems. A control station? Some people long thought they are time signal stations without knowledge that the transmitted "time period" is not 1 second...." Interesting.
Anyway here's the sound clip- This is the 'beat' between the two of the 820 Hz sidebands, which is resolved on an AM receiver.
Loran is a current wide area navigation system that uses flight-time of narrow pulses on a frequency or 100kHz. The Russian Chayka system is extremely similar and is compatible.
This off-air recording is of a mix of stations audible in the UK